The invention relates to determining hot gas temperatures using a tunable laser, and particularly to determining high pressure, combustion gas temperatures in a gas turbine.
Combustion gas temperature in a gas turbine is difficult to determine accurately. The combustion gas is extremely hot, corrosive, turbulent and at a high pressure. Combustion gas temperature, e.g., turbine firing temperature (Tfire), is conventionally estimated based on factors such as exhaust gas temperature and compressor discharge gas pressure. This estimation of combustion gas temperature has a certain level of uncertainty. To compensate for this uncertainty, the combustion gas temperature is set at a temperature lower than would be needed if the combustion temperature where known to a greater uncertainty.
Combustion gas temperature affects the power output of a gas turbine. Power increases as the combustion gas temperature is increased. For example, increasing combustion gas temperature (Tfire) by 10 degrees Fahrenheit may increase power output by one megawatt (1 MW) for a two-hundred megawatt (200 MW) gas turbine. Reducing the uncertainty level should allow combustion gas temperature to be increased and yield a corresponding increase in power output of a gas turbine.
Spectroscopic measurements, e.g., laser measurements, have been proposed to accurately determine gas temperatures in a gas turbine. International Patent Application WO 2007/014960 describes a temperature measurement device measures absorption of laser light at wavelengths corresponding to oxygen in a combustion gas flow of a gas turbine. U.S. Patent Application Publication 2008/0289342 describes determining combustion temperatures by measuring absorption of laser light wavelengths corresponding to oxygen in the gas flow of a gas turbine.
The laser wavelengths at which to measure absorption due to the combustion gas should be selected to optimize the accuracy of calculation of the gas temperature. The wavelengths at which the absorption is measured are conventionally selected as corresponding to temperature-dependent transitions of a species in the gas. There are several available wavelengths at which absorption occurs due to temperature-dependent transitions of a combustion gas species. A method is needed to select a pair of wavelengths at which laser absorption is to be measured to obtain absorption line strength data for accurately calculating combustion gas temperature in a gas turbine.